<!DOCTYPE html>
<html>

<head>
  <meta charset="UTF-8">
  <meta content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no"
    name="viewport">
  <title></title>
  <style>
    * {
      padding: 0;
      margin: 0;
    }

    html {
      font-size: clamp(10px, calc(100vw / (375 * 0.1)), 20px);
    }

    div {
      width: 10rem;
      height: 10rem;
      background-color: #ccc;
      font-size: 1rem;
      line-height: 1;
    }
  </style>
</head>

<body>
  <div>
    <span>AAAA</span>
  </div>

</body>

</html>